Detailed Process Analysis of Biobased Polybutylene Succinate Microfibers Produced by Laboratory-Scale Melt Electrospinning

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Detailed Process Analysis of Biobased Polybutylene Succinate Microfibers Produced by Laboratory-Scale Melt Electrospinning polymers Article Detailed Process Analysis of Biobased Polybutylene Succinate Microfibers Produced by Laboratory-Scale Melt Electrospinning Maike-Elisa Ostheller 1,† , Naveen Kumar Balakrishnan 1,† , Robert Groten 2 and Gunnar Seide 1,* 1 Aachen-Maastricht Institute for Biobased Materials (AMIBM), Maastricht University, Brightlands Chemelot Campus, Urmonderbaan 22, 6167 RD Geleen, The Netherlands; [email protected] (M.-E.O.); [email protected] (N.K.B.) 2 Department of Textile and Clothing Technology, Niederrhein University of Applied Sciences, Campus Moenchengladbach, Webschulstrasse 31, 41065 Moenchengladbach, Germany; [email protected] * Correspondence: [email protected] † Both authors contributed equally to this work. Abstract: Melt electrospinning is widely used to manufacture fibers with diameters in the low mi- crometer range. Such fibers are suitable for many biomedical applications, including sutures, stents and tissue engineering. We investigated the preparation of polybutylene succinate microfibers using a single-nozzle laboratory-scale device, while varying the electric field strength, process throughput, nozzle-to-collector distance and the temperature of the polymer melt. The formation of a Taylor cone followed by continuous fiber deposition was observed for all process parameters, but whipping behavior was enhanced when the electric field strength was increased from 50 to 60 kV. The narrowest Citation: Ostheller, M.-E.; fibers (30.05 µm) were produced using the following parameters: electric field strength 60 kV, melt Balakrishnan, N.K.; Groten, R.; temperature 235 ◦C, throughput 0.1 mL/min and nozzle-to-collector distance 10 cm. Statistical analy- Seide, G. Detailed Process Analysis of sis confirmed that the electric field strength was the most important parameter controlling the average Biobased Polybutylene Succinate fiber diameter. We therefore report the first production of melt-electrospun polybutylene succinate Microfibers Produced by fibers in the low micrometer range using a laboratory-scale device. This offers an economical and Laboratory-Scale Melt Electrospinning. Polymers 2021, 13, environmentally sustainable alternative to conventional solution electrospinning for the preparation 1024. https://doi.org/10.3390/ of safe fibers in the micrometer range suitable for biomedical applications. polym13071024 Keywords: polybutylene succinate; fiber spinning; nonwoven; environmental sustainability; melt Academic Editor: spinning; fiber production; electrospinning; melt electrospinning; process development; biomedi- Francesco Boschetto cal applications Received: 8 March 2021 Accepted: 24 March 2021 Published: 26 March 2021 1. Introduction Biocompatible and biodegradable polyesters are increasingly important in biomedical Publisher’s Note: MDPI stays neutral procedures, especially when used as sutures, bone fixation devices, plates, stents and with regard to jurisdictional claims in screws, as well as tissue repair and tissue engineering matrices [1]. Polylactic acid (PLA) published maps and institutional affil- and polybutylene succinate (PBS) are widely available thermoplastic biopolymers that iations. could replace petrochemical polymers in the future [2]. Biobased polyesters for medical applications are typically used in the form of films [3] or scaffolds in tissue engineering [4]. They can be manufactured by salt leaching [5], extrusion [6] or electrospinning [7]. Electrospinning is an efficient method for the manufacture of fibers ranging in diame- Copyright: © 2021 by the authors. ter from a few micrometers to hundreds of nanometers [8–10]. Such fibers are beneficial Licensee MDPI, Basel, Switzerland. because they combine an enormous surface area with high flexibility [11]. They are partic- This article is an open access article ularly useful for tissue engineering because fibers can be spun directly onto the surface distributed under the terms and of another material [12]. Further applications beyond the sphere of biomedicine include conditions of the Creative Commons filtration and separation [13], as well as electronics and energy [14,15]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ The global microfibers/nanofibers market was valued at US$477.7 million in 2016 and 4.0/). electrospinning was one of the most widely-used techniques for the production of such Polymers 2021, 13, 1024. https://doi.org/10.3390/polym13071024 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1024 2 of 16 fibers [16]. The two major electrospinning methods are solution electrospinning and melt electrospinning, both of which involve the exposure of a liquid to a strong electric field as it flows through a capillary in order to draw out fibers [12]. The large potential difference (tens of kilovolts) applied to the liquid leads to the formation of a jet that undergoes whipping movements, which in turn cause stretching and ultimately the deposition of microscale or nanoscale fibers onto a collector [17]. Solution electrospinning uses polymer substrates dissolved in an organic solvent, which evaporates as the fiber is spun, whereas melt electrospinning uses a high-temperature molten polymer. The lower viscosity and higher electrical conductivity of polymer solutions enables solution electrospinning to produce thinner fibers than melt electrospinning. However, PLA and PBS must be dissolved in toxic solvents such as chloroform and dichloromethane, which can be carried over to the final product. To make the process more environmentally friendly, researchers are currently focusing on the use of more benign solvents such as formic acid and acetic acid [18,19]. Nevertheless, an additional solvent recovery step is required, increasing the overall process costs [12]. For example, the production of 1 kg of PLA fibers, when processed as a 10% solution, requires 10 L of solvent with only 90% recovered in a typical process [20]. In contrast, melt electrospinning does not require solvents, but the melt must be held at high process temperatures in order to facilitate extrusion [21]. Furthermore, the high viscosity and low conductivity of the melt yields fibers with a thicker average diameter than solution electrospinning [11]. Nevertheless, because melt electrospinning has no requirement for solvents, and therefore no need for a solvent recovery step, it is more cost effective and environmentally sustainable than solution electrospinning [22–26]. The wider adoption of melt electrospinning could help to reduce the environmental footprint of current industrial electrospinning processes for medical-grade fibers, but solution electrospinning remains the favored industrial process because it has already been scaled up (and is therefore more economical) and also produces finer fiber products [9]. Solution electrospinning with PBS and PLA has successfully yielded fibers with average diameters in the sub-micrometer range for biomedical applications such as wound dressings. Melt electrospinning has also been carried out with PLA, but the brittleness and low conductivity of the material hinder the production of nanofibers and their applications, and various machine and material modifications have been tested to overcome this [27–37]. Compared to PLA, PBS is more ductile with a lower glass transition temperature (below room temperature). Accordingly, modifications to improve the ductility of this polymer may not be necessary. A porous nonwoven PBS mesh with a low pore size has been produced by solution electrospinning and developed for filtration applications [31]. The evaluation of solution- electrospun microscale and nanoscale PBS fibers for biomedical applications confirmed that their high porosity and hierarchical structure offers sufficient mechanical properties for applications in wound healing and soft tissue engineering [4]. The PBS backbone has more polar functional units than PLA, which means that PBS may be more electrically conductive than PLA in its molten form. But, despite the advantages set out above, melt electrospinning with PBS has, to the best of our knowledge, not been attempted before. Here we report the preparation of the first melt-electrospun PBS microfibers using our single-nozzle laboratory-scale device. We investigated the influence of four different process parameters on fiber diameter: temperature, electric field strength, throughput and nozzle-to-collector distance. We measured the effect of temperature on the viscosity and electrical conductivity of PBS melts. We also examined the thermal properties of the polymer, its susceptibility to degradation during processing and the influence of process parameters on fiber crystallinity. Finally, statistical analysis was used to predict the parameter with the greatest influence on the fiber diameter. 2. Materials and Methods 2.1. Materials Melt electrospinning was carried out using PBS fiber-grade resin (FZ78TM) produced by the polymerization of biobased succinic acid and 1,4-butanediol. The manufacturer Polymers 2021, 13, x FOR PEER REVIEW 3 of 18 2. Materials and Methods 2.1. Materials Polymers 2021, 13, 1024 Melt electrospinning was carried out using PBS fiber-grade resin (FZ78TM) produced3 of 16 by the polymerization of biobased succinic acid and 1,4-butanediol. The manufacturer (MCPP, Düsseldorf, Germany) reported the following specifications: a melt flow rate of 22 g/10 min at 190 °C and using a weight of 2.16 kg, and a crystalline melting temperature of(MCPP, 115 °C. Düsseldorf, The polymer
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